Ionic compounds generally form crystals composed of positively charged cations and negatively charged anions which pull electrostatically on each other. For example, when the ionic compound NaCl is dissolved in water, water molecules surround the Na+ ions (cations) and the Cl− ions (anions) that make up the crystals, turning each ion into a large overall ion. Because the large ions that form in this way are unable to approach each other, the electrostatic interactions therebetween weaken, enabling the cations and anions to move freely throughout the solution as particles carrying their respective charges.
In this way, ionic compounds dissolve in water and various other liquids to form liquids that conduct electricity, i.e., electrolytic solutions. Electrolytic solutions prepared by dissolving an ionic compound in an organic solvent are commonly used in nonaqueous electrolyte batteries and capacitors.
When NaCl is subjected to a rise in temperature and thermal motion is activated to such an extent as to overcome interactions between the ions, this compound itself becomes a liquid capable of conducting electricity. In the case of NaCl, the transformation of this solid into a liquid occurs at an elevated melting point of 800° C. An ionic compound or salt thereof that has melted in this way is generally called a fused salt.
The chemical species present in fused salts are all charge-bearing cations or anions; no neutral atoms or molecules are present. Accordingly, in a fused salt, elements that cannot be obtained from ordinary aqueous electrolytic solutions because the reducing power or oxidizing power with respect to water is too strong (i.e., metals such as alkali metals, aluminum and rare-earth elements, and non-metals such as fluorine) can be electrolyzed and thereby obtained in uncombined form. This is the principal industrial application of fused salts.
Among the above-described fused salts, there are some which remain in a liquid state even at room temperature and do not solidify at very low temperatures. Such fused salts which remain in a liquid state at room temperature or below are referred to in particular as “room-temperature fused salts” or “ionic liquids.”
These ionic liquids have a number of characteristics, including: (1) a vapor pressure that is either nonexistent or very low, (2) non-flammability or fire-retarding properties, (3) ionic conductivity, (4) a higher decomposition voltage than water, (5) a broader liquid temperature range than water, and (6) handleability in air.
Such characteristics are put to good use by employing these ionic liquids as novel electrolytes capable of being utilized at room temperature or lower in a variety of applications, including the electrodeposition of metals or alloys, electrolytic plating baths, electrolytes for energy-storing electrochemical devices, and as solvents for organic synthesis.
Of these applications, ionic liquids having a minimal water content are required particularly for use as nonaqueous electrolytes or as organic solvents in reactions that must be conducted in a water-free system.
Quaternary ammonium salts are generally prepared by the quaternization of a tertiary amine with, for example, an alkyl halide, a dialkyl sulfate or a dialkyl carbonate. Moreover, when changing the anionic species, synthesis is carried out via the quaternary ammonium hydroxide by neutralization with an inorganic acid or an organic acid. The quaternary ammonium carbonate or hydroxide salt used as the starting material for obtaining a quaternary ammonium salt by such a method is generally available commercially in the form of an aqueous solution or an alcohol solution. To reduce the water content therein, use has typically been made of a process in which the aqueous or alcohol solution is evaporated to dryness and the quaternary ammonium salt is removed as a solid, then is purified by recrystallization.
However, when a quaternary ammonium salt is industrially evaporated to dryness, it takes a long time to completely eliminate moisture, in addition to which the quaternary ammonium salt undergoes thermal degradation during heating.
A number of processes have been developed to avoid such problems, including a method in which, instead of removing the moisture completely, a solvent such as ethanol is added and concentration and recrystallization are carried out to lower the moisture content (JP-A 2001-106656), methods that involve storing the electrolyte or organic solvent produced in a low-humidity environment (JP-A 10-116631, JP-A 10-64540), a method in which a solute containing residual moisture is dissolved in an organic solvent and an inert gas is bubbled through the resulting solution to remove the moisture (JP-A 10-338653), and methods involving the use of desiccants.
However, moisture removal by recrystallization, drying with a desiccant or bubbling through an inert gas relies on resources such as alcohols, molecular sieves or nitrogen gas which are either difficult to reuse or for which a method of reuse must be established. Hence, moisture removal in this way is poorly suited for mass production. On the other hand, in methods involving storage in a low-humidity environment, it takes quite a long time to lower the moisture level.
In the case of quaternary ammonium salts which are solid at ambient temperature, such as (CH3)4N.BF4 and (CH3)3(C2H5)N.BF4, the moisture content can be lowered to about 10 to 20 ppm by vacuum drying crystals of the salt. However, in attempts that have been made to lower the moisture content in ionic liquids of relatively high viscosity using the above techniques, the lowest moisture content achieved has been about 200 ppm, and this was by dehydration carried out on a small amount of ionic liquid in a laboratory setting. When a large amount of ionic liquid is dehydrated at one time in a factory, for example, the percent reduction in moisture content worsens dramatically.
By making use of a method in which first the ionic liquid (CH3)(C2H5)2(CH3OC2H4)N.BF4 is dissolved in an organic solvent, then nitrogen gas is bubbled through the solution, the water content in the solution can be lowered to about 50 ppm. However, the salt concentration is difficult to adjust, making this approach inappropriate in cases where a specific concentration is required.
When an ionic liquid or a solution prepared by dissolving an ionic liquid in an organic solvent is used as a liquid electrolyte in an electrical storage device such as a lithium ion secondary cell or an electrical double-layer capacitor, to ensure the polar properties and longevity of the electrodes, “it is desirable for the moisture within the solvent to be lowered to less than about 30 ppm” (Denki Kagaku 48, No. 12, 665–671 (1980)). Hence, liquid electrolytes that have been subjected to a high degree of dehydration are required.
The present invention was conceived of in light of the above circumstances. One object of the invention is to provide highly dehydrated ionic liquids. Another object of the invention is to provide a method of dehydrating ionic liquids. Further objects are to provide electrical double-layer capacitors and secondary cells which use such ionic liquids.